The present invention relates to a kickdown system for an infinitely variable belt-drive transmission for a vehicle. U.S. Pat. No. 4,369,675 discloses a control system for an infinitely variable transmission.
The infinitely variable belt-drive transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley depending on driving conditions. The system is provided with an oil pressure regulator valve and a transmission ratio control valve. Each valve comprises a spool to control the oil by shifting the spool.
The primary pulley has a Pitot pressure generating device for producing Pitot pressure dependent on engine speed. The Pitot pressure is applied to one axial end of the spool of each valve to bias the spool. On the other hand, the actual transmission ratio is detected by the axial position of the movable conical disc of the primary pulley which represents the running diameter of the belt on the primary pulley. The position of the movable conical disc is transmitted to the other axial end of the spool of the pressure regulator valve by a rod and a link.
The spool of the transmission ratio control valve is shifted in dependency on the opening degree of a throttle valve of an engine and on engine speed to control the amount of oil supplied to the servo device of the primary pulley so as to control the transmission. The spool of the oil pressure regulator valve is shifted in dependency on the engine speed, the pressure in the servo device of the secondary pulley and on the transmission ratio to regulate the pressure of the line oil. Thus, the transmission ratio can be smoothly and infinitely varied.
In the system, when the accelerator pedal is fully depressed for kickdown, the transmission ratio control valve is operated in accordance with the full opening of the throttle valve so as to drain the oil in the servo device of the primary pulley, so that the transmission ratio is changed to a large ratio, that is a low speed state. The amount of shifting of the movable conical disc of the primary pulley is applied to the pressure regulator valve by the rod and link, so that the pressure of the line oil is increased to enhance the change-speed operation. However, the pressure regulating operation is the same as the operation at ordinary deceleration, and hence the kickdown is not quickly operated as described below.
The line oil pressure is determined by the relation between the Pitot pressure and the position of the rod dependent on the transmission ratio. On the other hand, since the engine speed increases, the Pitot pressure increases which acts to oppose the increase of the line oil pressure. FIG. 1 shows the relationship between the line oil pressure and the transmission ratio under the condition of a constant opening degree of the throttle valve. Accordingly, the line pressure does not rise quickly, and the response of the system to the kickdown operation is slow.
In order to improve the response of the system, two methods may be proposed. One of the methods is to quickly drain the oil in the servo device of the primary pulley, and the other is to increase the pressure in the servo device of the secondary pulley to a higher value than the ordinary deceleration. However, if the pressure in the servo device in the primary pulley quickly decreases, the belt tends to slip on the pulley.